Pipes used in the production and transportation of flowable media are subject to corrosion and plugging. An example of such a pipe is oil pipe which is generally large and for reasons of economy is manufactured from carbon steel rather than more expensive corrosion resistant alloys. Corrosion is induced by the hot underground environment in which down-hole pipes convey oil from deeply buried deposits to the earth's surface. Materials such as water, sulfur, sulfur dioxide, carbon dioxide, present in the oil typically make it acidic causing corrosion of the interior surface of the pipe. Even at cooler temperatures, transportation pipelines that extend for long distances close to the earth's surface experience the effects of corrosion because of the long contact times involved. Corroded pipes are difficult and expensive to replace.
Plugging occurs when organic materials soluble in the oil at high temperatures of the oil deposit become insoluble as the oil cools during the rise through a pipe to the earth's surface. Plugging may particularly be a problem for offshore wells because the oil is subjected seawater temperatures. The resultant insoluble materials, such as asphaltenes and paraffin waxes, tend to plate out of the oil at the high temperature of the oil deposit on the interior surface of the pipe, restricting the oil flow and eventually plugging the pipe. Also, soluble inorganic material, commonly referred to as scale and generally comprising calcite and/or barite, present in the oil or in the presence of salt water associated with conveying of oil from underground of subsea deposits, are present in the oil at the high temperature of the oil deposit. Plugging also occurs during long distance conveying of the oil through pipelines. Plugging requires that production or transportation cease while the pipe is cleaned out either by mechanical scraping (pigging), chemical treatment or hot oiling. Such cleaning reduces productivity and involves large maintenance costs. Similar problems occur for pipes used in the manufacture and transportation of corrosive chemicals in the chemical processing industry.
Moreover, the environment exterior to the pipes in which oil well pipes are used will contribute to such pluggage and corrosion in oil pipes. For instance, when a pipeline is installed offshore on the sea floor it may have high spots and low spots due to the undulation of the sea floor. In the low spots water may accumulate on the interior of the pipe. This water may come from hydrostatic testing of the pipeline or water entrained in the fluids carried in the pipeline. Such water may cause corrosion if it penetrates the pipe. Also, the water may contain carbonic acid or hydrochloric acid. And occasionally oil and gas may contain small amounts of corrosive gases such as carbon dioxide and hydrogen sulfide. When either of these gases are dissolved in water, acid is created that may attack the surface of the pipe. All of these causes may contribute to the failure of the pipe.
Solutions have been proposed in the oil industry for preventing corrosion and pluggage by coating oil pipes on their interior surface and their exterior surfaces, see, for example, U.S. Pat. No. 6,397,895 to Lively. This patent discloses the use of a phenolic primer in combination with an insulating layer and an abrasion resistant layer formed of an epoxy ceramic on the interior of a pipe. However, such epoxies do not provide a particularly non-stick surface to the oil. Lining the interior surface of oil well pipes with a fluoropolymer, such as polytetrafluoroethylene (PTFE), for example, as disclosed in EP 0 1910 092 to Mannesman Akt is known. Pope et al. in U.S. Pat. No. 3,462,825 have previously described manufacturing a pipe with a fluoropolymer liner. Such fluoropolymer linings present a non-stick surface to the oil. However, because of this non-stick property, these linings do not adhere particularly well to the interior surface of the pipe. In addition, the varying conditions of temperature, pressure and even mechanical contacts can cause such linings to separate from the interior surface, leading to loss in corrosion and possibly even non-stick protection if the coating or lining ruptures. However, at this time there does not exist a commercially attractive option for reducing pluggage and corrosion in pipes.
Thus, there remains a need for solving the problems of corrosion and plugging that occurs in pipes conveying flowable media, especially oil pipes, whether used in oil wells or for oil conveying. What would be desirable is a pipe system which is tailored to the particular environment in which the pipes are used, where the interior surface of such pipes resists the deposit of insoluble organic materials and has resistance to the corrosive effects of acids, and the exterior surface insulates the pipes in certain harsh environments. Further there is a desire that the interior and exterior surfaces be durable lasting for many years in harsh environments.